When magnesium or a magnesium alloy is melted for casting, a protective gas is used as an atmospheric gas in a melting furnace in order to prevent the molten metal from being oxidized and vaporized.
The protective gas is made into a gas (cover gas) that covers the molten metal in the melting furnace. A mixed gas obtained by diluting sulfur hexafluoride with a base gas such as an inert gas may be used as the cover gas (for example, Patent Document 1).
By using the mixed gas, it is possible to reduce the oxygen concentration in the melting furnace and to prevent the molten metal from being oxidized (burning). Further, since a film composed of reaction products of sulfur hexafluoride and a metal (magnesium, etc.) is formed on the surface of the molten metal, it is possible to prevent the molten metal from being vaporized.
Patent Document 1: Japanese Translation of International Application No. 2000-501653
However, the sulfur hexafluoride employed in the prior art is designated as a greenhouse gas (greenhouse coefficient: 23900), and a substitute has been sought in view of environmental safeguards.